The present invention relates to electron microscopes. It has utility both in scanning electron microscopes wherein a beam is scanned through a raster across a sample, as well as such microscopes which are used for point-to-point analysis. The present invention has particular utility in scanning electron microscopes used in the cathodoluminescent mode for biological applications. In the past, cathodoluminescent studies have been impeded by the need for stains used on the samples which are not damaged by the incident beam. The low levels of cathodoluminescent emission normally associated with biological samples has made it difficult to detect the resulting cathodoluminescence due to limitations on the capabilities of existing systems.
Typically, in current electron microscopes, mirrors are used adjacent the sample to collect the optical luminescence emitted by the sample. The mirrors collect the light and focus it to the input of an optical bundle or light pipe which transmits the light from within the vacuum system to a photomultiplier tube located outside the vacuum system. Although attempts have been made to design mirrors capable of collecting luminescence over a region extending 2.pi. steradians, in practice, systems have achieved somewhat less than that which is theoretically possible. Further, whatever light is collected reduces in intensity as it is transmitted through the light pipe. Light pipes are known to attenuate light intensity by as much as 60 percent per foot of length of the light pipe.
The present invention is directed to an electron microscope which uses a photodiode for sensing optical luminescence. Although the illustrated embodiment includes an electron microscope with a scanning beam, persons skilled in the art will readily appreciated it that the invention may be used for point-to-point analysis. Further, the term "electron microscope" is intended to be broad enough to include both beams of electrons and beams of other charged particles or ions.
The photodiode preferably is of the PIN type (that is, a PIN diode structure, as is known in the art, has a thin P-type diffusion in the front and an N-type diffusion into the back of the wafer of a very high resistivity silicon. The high resistivity material between the P-type and N-type diffusion is called the intrinsic region or I-layer. This optimizes both short- and long-wavelength response at low reverse bias. These diodes have a better spectral response than photomultiplier tubes, thereby extending the useful sensing range.
In the illustrated embodiment, the sensing photodiode and its feedback resistor and capacitor, which are associated with a feedback amplifier matched to the photodiode by the manufacturer, are mounted in a metallic cylindrical housing which is secured to the sample base beneath the sample in the transmitted mode or above and to the side of the sample in the emitted mode. Thus, the photodiode itself senses the optical luminescence directly adjacent the sample. This eliminates complex and expensive collection subsystems, and it further has an advantage in that the optical signal is converted to an electrical signal immediately adjacent the sample. Hence, an electrical signal, rather than an optical signal can be transmitted through the vacuum housing to the display circuitry, thereby obviating the problem discussed above of attenuation when an optical system is transmitted through a light pipe.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.